In the Fischer-Tropsch reaction synthesis gas is reacted in the presence of a catalyst to give a hydrocarbon mixture having a relatively broad molecular weight distribution. This product comprises predominantly straight chain saturated hydrocarbons which typically have a chain length of more than 5 carbon atoms. The reaction is highly exothermic and therefore heat removal is one of the primary constraints of all Fischer-Tropsch processes. This has directed commercial processes away from fixed bed operation to slurry systems. Such slurry systems employ a suspension of catalyst particles in a liquid medium thereby allowing both the gross temperature control and the local temperature control (in the vicinity of individual catalyst particles) to be significantly improved compared with fixed bed operation.
Fischer-Tropsch processes are known which employ slurry bubble columns in which the catalyst is primarily distributed and suspended in the slurry by the energy imparted from the synthesis gas rising from the gas distribution means at the bottom of the slurry bubble column as described in, for example, U.S. Pat. No. 5,252,613.
However the above processes require a synthesis gas stream to be continuously fed to the slurry when the catalyst is at elevated temperatures and pressures. This ensures that the catalyst is adequately agitated, avoids agglomeration of the catalyst and prevents the catalyst from settling to the bottom of the reactor. Any agglomeration or settling allows hot spots to develop within the catalyst which results in sintering and/or localized polymer formation which deactivate the catalyst. Consequently, when the Fischer-Tropsch process is initiated the synthesis gas reactant stream is used to agitate the catalyst as the temperature rises to the desired reaction temperature and subsequently when the Fischer-Tropsch process is shut down the catalyst is allowed to cool whilst a flow of the synthesis reactant gas stream is maintained thereby maintaining the level of agitation.
It has recently been found that a Fischer-Tropsch process may be operated by contacting synthesis gas with a suspension of a catalyst in a liquid medium in a system comprising at least one high shear mixing zone and a reactor vessel where suspension comprising the particulate Fischer-Tropsch catalyst suspended in liquid hydrocarbons is recycled back to the high shear mixing zone(s). This process is described in WO 0138269 (PCT patent application number GB 0004444) which is herein incorporated by reference.